This application claims the priority of German Patent Application, Serial No. 101 49 794.6, filed Oct. 9, 2001, pursuant to 35 U.S.C. 119(a)-(d), the disclosure of which is incorporated herein by reference.
The present invention relates to a device for measuring a motion of a moving body, and more particularly to a motion sensor capable of inductively measuring the velocity and acceleration of a moving body.
A motion sensor for inductively measuring the velocity and acceleration of a moving body is known, for example, from German Pat. No. DE 37 30 841 A1. A primary time-independent magnetic field that passes through a disk orthogonally to the direction of motion, is produced in a locally confined partial area of the disk near the edge of the rotating, electrically conducting disk that forms the moving body. For producing the primary field, two opposing permanent magnets are provided along an air gap through which the disk extends. These permanent magnets are also magnetically short-circuited on the sides facing away from the disk by a yoke made out of a magnetic material, for example iron, so as to form a closed magnetic circuit. The primary magnetic field of the permanent magnets induces in the moving disk locally electrical eddy currents which in turn induce a counteracting magnetic eddy current field. A Hall effect sensor or another magnetic field sensor, for example a magneto-resistive sensor, is provided on both sides of the gap for measuring the magnetic flux density produced by the eddy currents. The magnetic field sensor can determine the tangential velocity or the angular velocity of the disk. Each of the two magnetic field sensors is arranged in a gap of a corresponding flux collector ring made of magnetic material, for example iron, and also in the air gap between the two permanent magnets. Each of the flux connector rings defines a magnetic flux path in form of a loop which extends parallel to the disk or to the rotation plane of the disk and perpendicular to the primary field. Each of the flux connector rings has a linear segment which extends between the corresponding permanent magnets and the disk, wherein the gap with the magnetic field sensor is formed in the center of the segment, and a second U-shaped segment which is connected with the first straight segment and complements the first linear segment to form a closed flux path, with the U-shaped second segment projecting outwardly from the gap. The second segment of each of the two flux connector rings is surrounded outside the gap by a corresponding detector coil. These two detector coils measure the temporal variation of the flux density produced by the eddy currents and thereby provide a measurement signal for the temporal change of the tangential velocity or the rotation speed, and also for the acceleration or the rotational acceleration speed of the disk. Both the magnetic field sensors and the detector coils are oriented so as to measure the flux of the eddy current field which extends tangentially to the motion direction. According to DE 37 30 841 A1, both the magnetic field sensor and the induction coils are arranged closer to the moving body than the two permanent magnets and, on the other hand, as viewed in the motion direction, at the same height as the partial region in the moving body through which the primary magnetic field passes.
The device described in DE 37 30 841 A1 measures the tangential component of the measuring magnetic field in the air gap between the permanent magnets. However, since the tangential component can be accurately measured only at a relative large distance from the moving body, the permanent magnets in this prior-art device have to be arranged quite far from each other. As a result, the applied magnetic field becomes relatively small due to demagnetizing effects, resulting in a correspondingly small useful signal.
It would therefore be desirable and advantageous to provide an improved device for measuring a motion of a moving body to obviate prior art shortcomings.
According to one aspect of the present invention, a device for measuring a motion of an moving body (or: moved object) which is electrically conducting at least partially in a certain region includes magnetic field generating means (or: magnetic field sources) for producing a magnetic field (or: exciter field) which induces electric currents in the electrically conducting area of the moving body depending on the motion of the moving body, and at least one measuring device (or: detection device) for measuring a measuring magnetic field (or: measuring induction field) induced by the electric currents in the electrically conducting area of the moving body as a measure for at least one motion parameter of the moving body. The measuring device(s) measure(s) the measuring magnetic field at one location, where the field is at least approximately parallel to the motion direction. The magnetic field generating means are located closer to the moving body than the measuring device.
The term magnetic field or measuring magnetic field is used both for the traditional magnetic fieldxe2x80x94as used in the physical sciencesxe2x80x94as well as for a magnetic induction field (or: the magnetic flux density) or a magnetic flux, or a combination thereof. The measurement or evaluation of the measuring magnetic field also includes measuring or evaluating its temporal change or another function of the measuring magnetic field. For measuring the moving body in the magnetic field, only the relative motion between the moving body and the magnetic field is important. Accordingly, the moving body can be stationary relative to a pre-determined reference system, in particular the earth surface or to a machine part, and the magnetic field can be moved relative to the reference system or the magnetic field can be stationary relative to the reference system or the moving body can be moved relative to the reference system. The motion of the moving body is generally arbitrary and can be a translation, for example a linear motion, or a rotation, as well as a combination of a translation and rotation.
The invention is based on the concept, that the tangential magnetic field of the electric currents induced into moving body during its motion in the applied magnetic field of the magnetic field generating means can be measured with one or several measuring devices, and that these measuring devices can be to placed at a greater distance from the moving body than the magnetic field generating means. The invention is based on the observation that the tangential measuring magnetic field is relatively homogeneous or uniform, so that the distance between the measuring device and the moving body can be varied over a wider range than when measuring the vertical measuring magnetic field, which produces only relatively small changes in the measurement signal. Accordingly, the device of the invention has a greater installation tolerance for the measuring device(s) and/or a greater tolerance with respect to measurement deviations or unevenness of the surface of the moving body. The invention is based on another observation that measuring the tangential measuring magnetic field at a greater distance than the distance of the magnetic field generating means for the moving body produces an improved useful signal.
According to an advantageous embodiment, at least one measuring device includes at least one induction coil for measuring the measuring magnetic field, which in general has a coil axis which is at least approximately perpendicular to the magnetic field of the magnetic field generating means and/or at least approximately parallel to a motion direction of the moving body.
For guiding the measuring magnetic field through the induction coil (s) or also as a carrier for the coil winding, each induction coil generally has its own magnetic flux conduction body or coil core. Moreover, at least one termination element can be arranged on the end faces of each induction coil not only for guiding the flux, but also for facilitating mounting of the coil.
According to another advantageous embodiment, the magnetic field of the magnetic field generating means pass through the electrically conducting region of the moving body in at least one spatially confined partial area or in at least two spatially confined areas that are preferably spaced apart in the motion direction.
This arrangement of the measuring device relative to this partial area can be selected in different ways.
In one embodiment, at least one measuring device is arranged so that its projection onto the moving body in a projection direction that is parallel to the magnetic field of the magnetic field generating means or perpendicular to the motion direction of the motion body is located at least partially within this partial area. Advantageously, the measuring device can be arranged in the flux path of a measuring magnetic field extending around only one magnetically energized partial area in the moving body.
In another alternative embodiment, at least one of measuring device is arranged so that its corresponding projection is essentially entirely located outside each of the partial areas of the moving body through which the magnetic field of the magnetic field generating means passes. This second embodiment is preferred if at least two spatially separated, magnetically energized partial areas are provided in the moving body and the measuring device is therefore arranged with an offset to the partial areas and in particular between the partial areas. The projection of each measuring device onto the moving body in a projection direction that is parallel to the direction of the magnetic field generating means or perpendicular to the movement direction of the moving body is then located mainly between the two partial areas in the moving body through which the magnetic field of the magnetic field generating means passes.
The projection of at least one and preferably each measuring device onto the moving body has therefore the same spacing at least in the motion direction from an edge of the partial region through which the magnetic field of the magnetic field generating means passes, i.e., the measuring device is therefore arranged in the center of the partial region or at least between two partial regions.
Preferably, one and preferably each of the measuring device is formed or arranged in the motion direction substantially symmetrical to the magnetic field generating means, meaning that the measuring device is formed or arranged essentially symmetrical to the magnetic field generating means.
The central or symmetric arrangement of the measuring device(s) has the advantage that the measuring magnetic field tangential to the motion direction is greatest in an area above and below the partial area, thereby achieving a high sensor or measurement sensitivity and/or a large useful signal. The symmetry can be further increased by forming each measuring device in the motion direction essentially symmetric to the magnetic field generating means or to a common symmetry plane oriented perpendicular to the motion direction, i.e., if an induction coil is used, in particular with respect to its effective cross-section and the number of turns as well as the employed material.
According to another embodiment of the device, the magnetic field of the magnetic field generating means is oriented in a substantially identical direction in both partial areas of the moving body, in particular perpendicular to the motion direction of the moving body. Moreover, the magnetic field is homogeneous and/or time-independent.
Typically, the magnetic field of the magnetic field generating means is oriented substantially perpendicular to motion direction in both partial areas of the moving body. The magnetic field of the magnetic field generating means, in particular with a suitable design of the magnetic flux conducting means, is at least approximately orthogonal to the tangential measuring magnetic field in the region of the measuring device(s), and is therefore not measured by the measuring device or can be filtered out. Accordingly, variations in the applied magnetic field caused by potential temperature changes are eliminated in the measurement signal of the measuring device, keeping the overall temperature drift and noise small.
In a particularly advantageous embodiment of the invention, magnetic field conducting means (or: flux collectors, magnetic flux guiding means, yoke) are associated with the magnetic field generating means for conducting the magnetic field of the magnetic field generating means, wherein the magnetic flux guiding means preferably form a closed magnetic circuit (or: magnetic flux path).
According to another advantageous embodiment, magnetic flux conducting means for conducting the measuring magnetic flux are associated in addition or alternatively with at least one measuring device, with the many flux conducting means forming at least one closed magnetic circuit.
The aforedescribed magnetic flux conducting means guide the magnetic flux of the magnetic field generating means and/or the measuring magnetic flux in a predetermined fashion. The magnetic flux conducting means can be used with the magnetic field generating means to concentrate the magnetic field, to reduce stray fields and to increase the field strength in the energized region of the reference body. Conversely, the measuring device can employ additional magnetic flux conducting means to increase the effective permeability. This can enhance the useful or measurement signal of the measuring device.
The magnetic flux conducting means for the magnetic field generating means and the magnetic flux conducting means for the measuring device(s) preferably form independent magnetic field paths. By providing separate magnetic flux conducting means for the magnetic field generating means and the flux conducting means for the measuring device(s), and separate magnetic flux conducting means for the measuring device, their functions can be optimized and different geometries and materials can be used.
Alternatively, the magnetic field generating means and the measuring device can have at least partially common magnetic flux conducting means.
It has to be observed that the eddy current losses and/or thermal noise can be kept quite low by properly selecting the maternal and configuring the magnetic flux conducting means. The magnetic flux conducting means can be made of the maternal having a low specific electric resistance and/or can have lamellar structures or slots for reducing eddy currents. The material for the magnetic flux conducting means is generally magnetically conducting and/or soft magnetic. When using separate magnetic flux conducting means for the magnetic field generating means for the measuring device, the magnetic flux conducting means for the magnetic field generating means to have preferably a greater permeability than the magnetic flux conducting means for the measuring device, because the intermediate space of the magnetic field generating means, through which the moving body moves, has to be bridged by the exciter magnetic field. In addition, the magnetic flux conducting means of the measuring device can also be formed of a material with lower eddy current losses than the material for the magnetic flux conducting means of the magnetic field generating means. For example, a material can be selected for the magnetic flux conducting means of the measuring device in the form of an iron-powder-containing material or a ferritic magnetic material. For the magnetic flux conducting means of the magnetic field generating means, an exemplary material can be soft iron with a high permeability.
In another advantageous embodiment, the magnetic flux conducting means for the magnetic field generating means guide the magnetic field of the magnetic field generating means at least approximately perpendicular to the motion direction of the moving body and/or at least in a plane that is oriented perpendicular to the motion direction of the moving body. This practicality eliminates a tangential component of the exciter field that could interfere with a tangential measurement of the measuring device.
The magnetic flux conducting means for the measuring device(s) preferably guide the measuring magnetic field in at least one plane that includes the motion direction of the moving body.
In another advantageous embodiment, the magnetic flux conducting means for the magnetic field generating means can direct the magnetic field of the magnetic field generating means into a plane which is essentially oriented perpendicular to the plane in which the magnetic flux conducting means for the measuring device conduct the measuring magnetic field. Accordingly, the magnetic field paths of the magnetic flux conducting means for the exciter field and the measurement field are orthogonal and therefore (nearly) independent of each other and practically do not to interfere with each other.
In yet another embodiment, at least one pair of measuring devices is provided which are arranged on different sides of the moving body. The two measuring devices can then be electrically connected so as to add the useful measuring signals of the two measuring devices and to at least partially compensate or eliminate the interfering signals by a common mode suppression (or differential suppression). Such interfering signals are produced in particular by changes in the distances between the measuring devices and the moving body which produce generally an interfering signal which increases with increasing rotation speed. The changes in the distance can be caused by the deviations of the moving body from a predetermined path, for example as a result of a tumbling or tilting motion. Since the measuring magnetic field on opposite sides of the moving body is generally oriented in opposite directions, the two measuring devices in this embodiment are generally connected with opposing electrical polarity.
Preferably, the two measuring devices are arranged directly opposite of each other so that their projections onto the moving body overlap, and/or the two measuring devices are essentially constructed in an identical manner. Each of these measures improves the suppression of interfering signals.
The magnetic field generating means include in general at least one pair or at least two pairs of permanent magnets that are mutually offset in the motion direction and have opposing magnetic poles (or poles of the opposite polarity), between which the moving body moves or is moveable.
In another embodiment, the magnetic field generating means and/or the magnetic flux conducting means and/or the measuring device and/or the magnetic flux conducting element and/or the termination element can be each secured to a single or to a common support and/or can be thermally coupled with the support. The support has then preferably a good thermal conductivity and can be made to in particular of a metal. The thermal coupling with the support which has preferably a good thermal conductivity, has the advantage that heat produced by eddy current losses are removed from the magnetically conducting elements, so that these can be effectively cooled.